A. Field of the Invention
The present invention relates to a method for replacing an existing refrigerant in a centrifugal compressor while maintaining or modifying the performance of the centrifugal compressor.
B. Description of the Related Art
Typically, centrifugal compressors are designed by selecting a gear arrangement which will provide the desired impeller speed and impeller Mach number when used in combination with a predetermined refrigerant. Impeller Mach number is the tip speed of the impeller divided by the acoustical velocity of the refrigerant. The acoustical velocity is the speed of sound in the refrigerant vapor.
A current problem which arises with centrifugal compressors is that commonly used refrigerants, such as R11, are fully halogenated chlorofluorocarbons (CFCs). Fully halogenated CFCs are a class of compounds which can adversely affect the environment, such as by depleting the ozone layer. Therefore, it is desirable to replace existing refrigerants, such as R11, with more environmentally acceptable refrigerants.
Suitable replacement refrigerants often do not have the same molecular weight as the existing refrigerant. Because the acoustical velocity of the refrigerant is related to the molecular weight of the refrigerant, the impeller Mach number will be altered if a refrigerant having a different molecular weight is utilized in the centrifugal compressor If the replacement refrigerant has a molecular weight higher than that of the existing refrigerant, the acoustical velocity of the replacement refrigerant will be lower than the acoustical velocity of the existing refrigerant. Therefore, the impeller Mach number will be higher with the replacement refrigerant than with the existing refrigerant. Alternatively, if the replacement refrigerant has a molecular weight lower than that of the existing refrigerant, the impeller Mach number of the centrifugal compressor will be lower with the replacement refrigerant.
Increasing or decreasing the impeller Mach number of the centrifugal compressor will place the operating point in an inefficient region of the compressor map. In other words, the centrifugal compressor will be placed in an inefficient operating condition.
Known ways to maintain centrifugal compressor performance while replacing the existing refrigerant are to alter the gear arrangement to enable the centrifugal compressor to operate efficiently while using the replacement refrigerant, or make structural modifications to the centrifugal compressor. For example, by changing the gear arrangement, the speed of the impeller can be slowed, thereby adjusting the impeller Mach number to provide better efficiency. Such methods are expensive and also have other disadvantages which will be discussed later.
There are also circumstances where it would be desirable to modify the performance of an existing centrifugal compressor. Currently, one way of accomplishing this is to make structural modifications to the centrifugal compressor, but, as discussed above in regard to maintaining compressor performance, this is an unsatisfactory method due to the cost, as well as the loss of the availability of the compressor.